This invention relates to a method of deriving a compressed facsimile signal from an original facsimile signal according to an all-white block skipping scheme. This invention is specifically adapted to a facsimile transmission system comprising analog modems.
It is well known that an original facsimile signal derived from an original text has redundant graphic or picture information. An excellent method of compressing the redundant information for digital transmission, is to resort to the run length encoding as called in the art. When the information-compressed signal is to be transmitted through a telephone line, digital modems are used for transmission at a rate of, for example, 4,800 or 9,600 bauds. Should an error occur even at a bit of the run length encoded signal during transmission from a transmitting end to a receiving end, serious degradation is inevitable on the text reproduced at the receiving end. The digital modems must therefore be highly reliable and are consequently expensive.
For analog transmission of facsimile signals, compression of graphic information is usually carried out by an all-white block skipping scheme or technique. According to the scheme, all white areas of the original text are skip-scanned in effect. As will later be described with reference to a few of several figures of the accompanying drawing. It has been impossible to obtain an excellent reproduced text by conventional all-white block skipping schemes. Even with an improved scheme, it has been impossible to insure continuity of black areas on the reproduced text. Furthermore, it has been infeasible to transmit the all-white block skipped facsimile signal through a carrier transmission line comprising a plurality of links because of such degradation in the continuity of black and/or white areas on the reproduced text.